In practice, a wireless communication transmitter emits energy in frequency regions other than those intended for the transmission. This emission, or leakage, of energy into nearby frequency regions generally relates to the operating point of the transmitter□ s power amplifier. The level of energy leaked, for example, increases when the power amplifier operates in its non-linear region, at higher power levels, due to intermodulation distortion. To circumvent this, it would be possible to design the transmitter circuitry to improve the linearity of the power amplifier. This will however come at the expense of significantly reduced efficiency, and consequently significantly reduced battery time. Accordingly, an effective method for achieving a pre-determined limitation on the amount of energy leaked entails reducing or □ backing off□ the maximum transmit power on the intended transmit channel from its nominal value.
The amount by which the maximum transmit power must be backed off, while also accounting for amplifier efficiency, depends on properties of the transmitted waveform (e.g., the modulation, spreading code, spreading factor, gain factors, or similar configurations of the waveform specified in one or more transport formats selected for the transmission). For some signals, these properties can be well quantified in terms of Cubic Metric (CM) or Peak-to-Average Power Ratio (PAPR), as described in e.g. 3GPP Technical specification TS25.101, Release 8. Such quantities, however, can be costly in terms of processing resources to compute quickly, making estimation of the required back-off upon a dynamic change in the properties of the transmitted waveform particularly problematic.
With various known approaches addressing this issue for single-carrier transmission, PAPR or CM can be pre-computed for all possible configurations of the transmitted waveform and the corresponding required back-off stored in a look-up table. This approach, however, proves more and more impracticable as the number of configuration possibilities increases, due to the size of the required look-up table. In multi-carrier transmission, for example, two or more separately modulated carriers, each occupying distinct frequency regions, are transmitted simultaneously. See, e.g., multi-carrier operation outlined for inclusion in 3GPP Rel.9, □ Dual-Cell HSUPA,□ 3GPP Work Item Description, RP-090014. When the configuration for each carrier is selected independently from that of the other carriers, the number of possible configurations of the compound waveform (and thereby the size of the required look-up table) may be several orders of magnitude greater than in single-carrier transmission.